The steadily increasing demand for energy in the form of gas and oil has required the drilling of wells at ever-increasing depths. The loss of a rotary bit, drill collar, or the lower portion of a drill string in these wells is very costly ("The Importance of Quality Tubular Inspections" by M. C. Moyer, et al., Oil & Gas Journal, Apr. 13, 1981). Because flaws occur in drill pipes and collars as a result of tensile, compressive, torsional and bending stresses encountered during the drilling operation, it is not uncommon for each length of drill pipe and each drill collar to be inspected prior to use in drilling a new well. Currently, the magnetic particle method is used for the inspection of drill pipe and collars. While experience has shown results obtained using the magnetic particle method are generally better than most, increased flaw-detection sensitivity is necessary, particularly in the threaded regions of tool joints, if costly failures are to be significantly reduced.
Referring more specifically to FIG. 1, a tool joint, indicated generally at 20, comprises a pin 21 of one drill pipe 22 threadably engaged within the box 23 of a second drill pipe 24 so that the end of the box 23 butts against the shoulder 25 of the pin 21. A thread indicated generally at 26 comprises a helical root 27 bordered by crests 28 and 29. Flaws in the threads of the tool joint 20 tend to occur in the last engaged threads of the pin 21 near the shoulder 25 or the last threads of the box 23 outside the engaged region. When the tool joint is new, failures occur primarily in the threads of the pin 21 rather than the threads of the box 23 because of the preloaded stress distribution and the presence of initiating sources such as tool marks, nicks, gouges, corrosion, etc. However, as the tool joint 20 is used in service, failures occur more frequently in the threads of the box 23 because of the wear on the outside diameter. Consequently, the threads of both the pin 21 and the box 23 must be inspected. Since the inspection activities must be conducted primarily at the drill site, adequate consideration must be given to the undesirable environment and operational conditions at the drill site.
As mentioned above, the magnetic particle method is used for the inspection of the threads of box and pin joint regions. This has been accomplished by magnetizing the region or providing it with a residual magnetic field and thereafter dusting fine particles of iron or iron oxides on the region to ascertain whether or not there are any defects of the type referred to above. However, this type of inspection has serious limitations inasmuch as the threads must first be cleansed of any oil, dirt, corrosion, or the like. Additionally, the inspection is very dependent on the visual acuity of the inspector.
U.S. Pat. No. 3,327,205 granted June 1967 to F. M. Wood, et al., discloses apparatus for inspecting the threaded ends of pipe which comprises a housing for securing the outer diameter of the pipe being inspected, a rotor adapted for circumferential rotation with respect to the central axis of the pipe, a search shoe moved longitudinally by the rotor with respect to the pipe and having a threaded portion for engagement with the threads of the pipe to be inspected. Means are mounted in the threaded portion of the shoe for sensing a transverse magnetic field. Consequently, the magnetic responsive means must be positioned in very close proximity to the root of the pipe thread on which the shoe rides. Although the accuracy of this apparatus surpasses that of the magnetic particle method, it still requires that the threads be cleansed so that the magnetic responsive means ride in very close proximity to the root of the pipe threads. Also, no mention is made of the magnetization field required to accurately identify flaws via the magnetic responsive means.